The present invention relates, in general, to a procedure for judging the quality of printed sheets. More particularly, the invention relates to a method for judging the quality of printed images on a printing carrier, such as paper, wherein a master image is obtained by first electronically scanning a master printed sheet, and thereafter scanning a multiplicity of proof sheets that have been judged to be acceptable by a human inspector to establish a subjective tolerance range for each of a plurality of image pixels. Subsequently, printed sheets are electronically reviewed and the ink density in selected pixels is compared with the stored values for those pixels. Sheets with ink densities within the allowable tolerance range are judged acceptable, while those with values outside the range are rejected.
Devices for judging the quality of printed sheets, or proof copies, are generally known; see, for example, European published unexamined patent application No. 0194331. The checking of print quality refers to the measuring of ink density values in measuring fields or areas selected for that purpose. Register deviations are checked by measuring special register faults and contrast marks for analyzing dot increases and contrast. Initially, these quality control checks could not be carried out over an entire printed sheet, but systems became available on the market which permitted such monitoring with the help of image exposure devices which project a printed sheet or a section of a printed sheet as a standing image on a monitor. Such standing images were stored in an image memory so that the printer could look at a selected still image of a printed sheet and could control the printed product on-line. Once it was known to do this, it became possible to compare later images derived from the printed products with the stored initial image and to produce signals representing any deviations, and such systems are now known.
However, it has been found such image comparison systems can result in excessive losses, for such systems operate to produce error signals whenever an image element comparison exceeds a predetermined error limit, represented by a defined difference between nominal and actual values. Such a rigid system of detecting errors is unusable in practice, since an exact correspondence of individual image elements between the "master" image and a printed image cannot be guaranteed during a printing run because of a variety of factors such as deformation of the print carrier, and the like. Such displacements of printed image elements can cause error signals in an automatic system even though the differences between the master image and the proof image are small, and would be judged acceptable by an inspector.
A difficulty with automatic image comparison systems, therefore, is that they may result in rejection of a printed image in cases where an inspector might consider the differences to be insufficient to cause an error signal. Tests have shown that visual examination of a printed image for the detection of printing errors is very subjective. When examining homogeneous surfaces; for example, unprinted sections, the human eye acts with the highest sensitivity to detect errors, and in such a situation, the slightest irregularities will be recognized immediately. On the other hand, the same irregularities within a printed section may not be recognized by a human inspector. However, both irregularities would cause an automatic inspection system to reject the printed image. Thus, there are significant limits to the use of automatic error detection systems for measuring irregularities in a printed product.